Process control method and apparatus for regulating temperature

ABSTRACT

A process control method comprises the steps of generating a first electrical signal representing a measured value of a process variable, generating a second electrical signal representing a desired value of said process variable, generating a third electrical signal, which changes at a prechosen rate corresponding to a desired rate of change of said measured value of said process variable, comparing said second electrical signal with said third electrical signal to generate a fourth electrical signal, applying said fourth electrical signal to cause said third electrical signal to tend towards equality with said second electrical signal, and comparing said first electrical signal with said third electrical signal to produce a process control signal therefrom. Apparatus for carrying out the method is also described as well as the application of the method to the control of the temperature of dyebath liquor.

United States Patent Nienstaedt et al.

[is] 3,692,986 1 Sept. 19, 1972 PROCESS CONTROL METHOD AND APPARATUS FORREGULATING TEMPERATURE Inventors: Robert 1 Nienstaedt, Skensved,Denmark; James William Bunting, Leamington Spa, England Assignee:Courtaulds Engineering Limited Filed: Jan. 5, 1971 Appl. No.: 104,112

US. Cl ..235/l5l.l, 236/46 Int. Cl. ..G05d 23/00 Field of Search..235/l5l.l, 150.1, 151; 236/46;

References Cited UNITED STATES PATENTS l/l967 7/1968 7/1959 l/l9673/1967 3,553,992 l/197l l-larbaugh ..235/l5l.1 UX

Primary Examiner-Joseph F. Ruggiero Attorney-Davis, l-loxie, Faithfull &Hapgood [57] ABSTRACT A process control method comprises the steps ofgenerating a first electrical signal representing a measured value of aprocess variable, generating a second electrical signal representing adesired value of said process variable, generating a third electricalsignal, which changes at a prechosen rate corresponding to a desiredrate of change of said measured value of said process variable,comparing said second electrical signal with said third electricalsignal to generate a fourth electrical signal, applying said fourthelectrical signal to cause said third electrical signal to tend towardsequality with said second electrical signal, and comparing said firstelectrical signal with said third electrical signal to produce a processcontrol signal therefrom. Apparatus for carrying out the method is alsodescribed as well as the application of the method to the control of thetemperature of dyebath liquor.

15 Claims, 2 Drawing Figures This invention relates to a method andapparatus for effecting control of changes in a process variable, andmore particularly for effecting such changes at prechosen rates,

One example of a process variable which the invention may be used tocontrol is temperature. In many manufacturing operations which involveheating or cooling treatments, it is desirable not only to apply acontrolled temperature, but also to ensure that the heating to and/orthe cooling from such a temperature is carried out at a pre-chosen rate.Moreover, it may be necessary to vary the rate of heating and/or coolingin a pre-chosen manner in order to achieve some desired property in anend product. Examples of processes in which such temperature control isdesirable include annealing treatments and textile dyeing operations.

In the latter, proper control of the dyeing operation normally requiresthat the dyebath liquor be brought to a precise temperature at a closelycontrolled rate, that it be held at the dyeing temperature for apredetermined period of time, and that it be subsequently cooled at thesame, or another closely controlled rate. This is necessary to ensureuniformity of shade between successive batches of fabric treated insuccessive dyeing operations.

Hitherto it has been difficult to achieve smooth stepless control ofprocess variable over extended periods of time, except by the use ofmechanical devices, such ascams, whose profile is not readily alterable.

According to one aspect of this invention, a process control methodincludes the steps of generating a first electrical analoguerepresenting a measured value of a process variable, generating a secondelectrical analogue representing a desired value of the same processvariable, generating a ramp function (as hereinafter defined) at apre-chosen rate corresponding to a desired rate of change in themeasured value of the process variable, comparing said ramp functionwith the second electrical analogue to generate a signal, applying saidsignal so as to cause the ramp function to tend towards equality withthe second electrical analogue, and comparing the first electricalanalogue with the ramp function to derive at least one process controlsignal therefrom.

Preferably the method includes the initial step of bringing the startingpoint of the ramp function to the same value as the analogue of themeasured process variable.

A process control apparatus according to a further aspect of thisinvention includes an electrical circuit comprising a first amplifier,(as hereinafter defined) one of whose inputs is connected to anelectrical signal representing a desired value of a process variable,and the other of whose inputs is connected to a first input of a secondamplifier, (as hereinafter defined) the other input to the secondamplifier being connected to an electrical signal representing ameasured value of the same process variable, a ramp function generator(as hereinafter defined) arranged to generate a ramp function at aprechosen rate corresponding to a desired rate of change in the measuredvalue of the process variable, the input of the ramp function generatorbeing connected to the output of the first amplifier and the directionof the ramp function being controlled thereby so as to tend to equalizesaid ramp function and the first signal, the output of the ramp functiongenerator being applied to the common input to the first and secondamplifiers, and means responsive to the output of the second amplifierto generate at least one process control signal.

Preferably the apparatus includes electric circuit means operable toconnect the output of the second amplifier to the ramp functiongenerator, whereby the output potential of the latter is initiallycaused to equal the second signal.

By amplifier in this specification we mean a highgain, high inputimpedance differential amplifier, for example, of the kind generallyknown as an operational amplifier. Amplifiers of this kind are availableboth in the form of integrated circuits and as assemblies of dis cretecomponents. By ramp function generator we mean a circuit which producesan electrical signal the voltage of which changes at a constant rate fora period immediately following a period in which the voltage remainsconstant or changes at another rate, without a transitional voltage stepbetween the periods. Hence, the term ramp function in this specificationmeans the signal generated by a circuit of the aforesaid kind.

One circuit capable of use as a ramp function gene rator is for examplethat disclosed in British Patent Specification No. 1 199150. Amodification of this particular circuit which is especially convenientfor use as a ramp function generator, is summarized below.

The modified circuit just referred to is an integrator circuit, and theinvention includes a process control apparatus embodying at least onesuch integrator circuit.

A process control method and apparatus according to this invention willnow be described, by way of example, with reference to the accompanyingdrawing, in which:

FIG. 1 is a block diagram of part of a process control apparatus, and

FIG. 2 is a schematic diagram of one practical form of the apparatus ofFIG. 1, in which the ramp function generator is an integrator circuit.

In FIG. 1, which is intended only to illustrate the basic principles ofthe invention, a first amplifier A has a first input 1, connected to anelectrical signal having a potential E,, which represents a desiredvalue of a process variable, and a second input 2 connected to theoutput 6 of a ramp function generator R. The output 3 of the amplifier Ais connected to the input 4 of the ramp function generator R, thepotential at the output 3 being designated E The output 6 of the rampfunction generator R is also connected to one input 7 of a secondamplifier A whose other input 8 is connected to a signal having apotential E and representing the measured value of the same processvariable. The potential of the signal generated by the ramp functiongenerator is designated E The output 9 of the amplifier A is connectedto means (not shown) which is responsive to its output potential E andserves to operate process control equipment (also not shown).

Both amplifiers are high-gain operational amplifiers connected in anopen loop mode, so that even a very small potential difference betweentheir inputs will result in an output potential the sign of which willdepend on the sign of the input potential difference, and.

the magnitude of which will depend on the difference between theoperating potentials supplied to the amplifier. Examples of suitableamplifiers which may be used are integrated circuit devices such as theTexas Instruments SN 74lP and the Fairchild p. A 709.

Therefore, the potential of the signal E (which may be derived in anyconvenient way, for example from a potentiometer or from a furtheroperational amplifier connected in the summing mode), when applied tothe input I of the amplifier A,, will result in the latter producing anoutput potential E, (which may be either positive or negative) dependingon whether the signal E is more or less than E the potential at theother input 2.

If the potential E equals the potential E,,, the output potential E willbe zero or very nearly so.

The potential E (when the ramp function generator R operates) isarranged to rise or fall at a pr'e-chosen rate corresponding to adesired rate of change in the measured value of the process variableunder control, the direction of the change being responsive to the signof the output potential E, of the first amplifier A,, such that thepotential E tends towards equality with the potential E When the outputpotential E,, of the first amplifier A, is zero then the potential E isconstant.

Because the potential E is also applied to one input of the amplifier Awhose other input is connected to the potential E the output potential Ewill be either positive or negative, depending on the sign of thedifference between the potentials E and E By suitably applying thepotential E to control the process variable from which the potential Eis derived, the circuit operates until the potential E falls to zero, atwhich time the potential E equals the potential E and also equals thepotential E and the output potential E, from the first amplifier A iszero. Thus, because the potential E is generated at a pre-chosen rate,the process variable under control can be brought to a desired value ata pre-chosen rate, and because the direction of the ramp function isarranged to depend on the sign of the potential difference between thepotentials E and E it does not matter whether the initial measured valueof the process variable is above or below the desired value.

In the practical circuit of FIG. 2, the ramp function generator isconstituted by an integrator circuit comprising the amplifier A, of FIG.1 and a further amplifier A which has a capacitor C, connected betweenits output 6 and one of its inputs 4, the other input being connected toa reference potential, in this case to earth which may be regarded aszero potential. For convenience, corresponding parts of both Figuresbear the same designations.

The amplifier A, may be a device of the kind previously referred to, butwhere the process being controlled demands that the process variable bechanged relatively slowly, or that it be controlled over an extendedperiod, for example throughout a period of several hours, we prefer touse an amplifier having a high degree of stability. One example of asuitable amplifier is the Analog Devices 40-1, which includes afield-effect input stage, and which exhibits the desired low-driftoperating characteristics.

A fraction of the output potential of the amplifier A, is tapped off ata potentiometer R and is connected by switches S S S and S, throughresistors R,, R R and R to the of 4 of the amplifier A the value of thecapacitor C, and the values of these resistors being chosen such thatthe rates of change of the potential E can be preselected correspondingto desired rates of change in the measured value of the process variablewhich is to be controlled. A switch S and a resistor R, enable theresistors R, to be by-passed, for a purpose which will be explainedlater. In like manner, a switch S and an associated resistor R enablethe input 4 to be connected to the output 9 of the amplifier A Theoutput of the latter is connected by a pair of diodes D, and D to apolarized relay H.

A fourth amplifier, A, has an input 10 connected to the potential E andhas its other input 1 1 connected to the potential E A capacitor C and aresistor R are connected in series between the input 10 and the output12 of the amplifier A whose output potential is denoted by E The meansof supplying the operating potentials for the various amplifiers,together with other associated circuitry is, for simplicity, not shown.

The operation of the integrator circuit formed by the amplifiers A, andA, may be summarized as follows.

On closing one (or more) of the switches S, the capacitor C, begins tocharge or discharge through the associated resistor, or resistors R, ata rate which is effectively determined by the time constant of thecombination of these components, and in a direction determined by thesign of the output potential of the amplifier A,. The potential Etherefore rises, or falls, as the case may be, until the circuitstabilizes at the point when the potential E is the same for practicalpurposes as the potential E1, supplied to the other input of theamplifier A,.

In practice, the values of the resistors R, and the capacitor C,, arechosen to give preselected rates of changes of the potential E inaccordance with desired rates of change in the measured value of theprocess variable under control, but a potentiometer could be used togive a manually variable rate of change, if so desired. The remainder ofthe circuit of FIG. 2 operates in the same way as that of FIG. 1.

However, one disadvantage of the basic arrangement of FIG. 1 is that theinitial value of the potential E may not be, and in practice usually isvery different from, the potential E This means that the ramp functionwill start to operate from some unknown point relative to the actualmeasured value of the process variable I represented by the potentialE,,.

This may be simply and effectively overcome by means of the switch S andits associated resistor R On closing the switch S the output potential Eof the amplifier A is connected, through the resistor R to the input 4of the amplifier A and these two amplifiers then form an integratorcircuit, in the same way as do amplifiers A, and A,.

By making the time constant of the combination of the capacitor C, andthe resistor R sufficiently small in comparison to that of the capacitorC, and the resistors R, this integrator circuit causes the potential E,to change rapidly until it equals the potential E On opening the switch5,, the potential E, from the amplitier A resumes control of the rampfunction which thus starts from the potential E In practice, this may beachieved by making the value of the resistor R much less than any of theresistors R Likewise, if it is desired at any time to cause the processvariable under control to change as quickly as possible to the desiredvalue represented by E the switch S may be closed, allowing thecapacitor C to charge rapidly through the resistor R the latter alsobeing made vary much smaller than any of the resistors R, In this caseof course, the potential E is driven rapidly towards the potential E inthe same manner that it can be driven towards the potential E by theswitch S In the circuit of FIG. 2, the output potential E of theamplifier A is always either positive or negative, with respect toearth, or to some arbitrary zero.

The diodes D, and D are back-connected so that the polarized relay IIwill change state, depending on the sign of the potential E The relaycontacts (not shown) may operate a conventional servo-control system inwhich one or more process controls are turned on and off, according towhether the potential E is greater or smaller than the potential E but amore convenient arrangement makes use of the amplifier A This developsan output potential E related to the potential E and that of the rampfunction E The output potential E is of the knownproportional-plus-integral kind, being an analogue of the potentialdifference between the potentials E and E This output potential E can bemade to have a specific relationship to the potentials E and E dependingon the values of the capacitor C and the resistor R and this can beparticularly useful for close control of certain process variables, suchas temperature.

The process control method and apparatus described in relation to FIGS.1 and 2 is particularly convenient for the control of temperature in adyeing operation because it enables the dyebath liquor to be brought toa precise temperature at a closely controlled rate from an initialtemperature which may be above or below the desired final temperature.

Furthermore, successive dyeing operations can be carried out under thesame conditions thereby ensuring good uniformity of shade betweensuccessive batches of fabric.

In the case of such temperature control processes, the relay outputcircuitry could be arranged to select heating or cooling, and the outputpotential E could then be arranged to control the exact amount of eitherof these to be applied to the process.

Further amplifiers may also be used in addition to the amplifiers A andA in order to compare the potentials E and E For example signals may beproduced when one of these potentials exceeds the other, indicating thatthe measured value of the process variable is above or below its desiredvalue, or a signal may be produced to indicate that the potentials areequal, showing that the measured value equals the desired value.

The potential E may be generated in any convenient manner, for exampleby means of a potentiometer. The potential E in the case of atemperature control process, may be generated by a thermocouple deviceor a resistance thermometer. If required, either or both of thepotentials E and E may be connected to the circuits of FIGS. 1 and 2 bybuffer amplifiers, and operational amplifiers of the types referred toearlier in rela-.

tion to FIG. 1 may be used for this purpose, for example by connectingthem in the well-known summing mode".

What is claimed is:

l. A process control method, comprising the steps of:

i. generating a first electrical signal representing a measured value ofa process variable,

ii. generating a second electrical signal representing a desired valueof said process variable,

iii. generating a third electrical signal, which changes at a prechosenrate corresponding to a desired rate of change of said measured value ofsaid process variable,

iv. comparing said second electrical signal with said third electricalsignal to generate a fourth electrical signal,

v. applying said fourth electrical signal to cause said third electricalsignal to change at the prechosen rate towards equality with said secondelectrical signal, and

vi. comparing said first electrical signal with said third electricalsignal to produce a process control signal therefrom.

2. A process control method according to claim 1, including a furtherstep of causing the third electrical signal to have an initial valuewhich is equal to the first electrical signal.

3. A method of controlling the temperature of a dyebath, comprising thesteps of:

i. generating a first electrical signal representing a measured value ofthe dyebath temperature,

ii. generating a second electrical signal representing a desired valueof the dyebath temperature,

iii. generating a third electrical signal, which changes at a prechosenrate corresponding to a desired rate of change of the measured value ofthe dyebath temperature,

iv. comparing said second electrical signal with said third electricalsignal to generate a fourth electrical signal,

v. applying said fourth electrical signal to cause said third electricalsignal to change at the prechosen rate towards equality with said secondelectrical signal, and

vi. comparing said first electrical signal with said third electricalsignal to produce a temperature control signal therefrom.

4. A process control apparatus comprising an electrical circuit having:

a means for generating a first electrical signal.

representing a desired value of a process variable, a means forgenerating a second electrical signal representing a measured value ofsaid process variable, a first operational amplifier, with an output anda first and second input thereto, said first input being connected tosaid first signal.

a second operational amplifier with an output and a first and a secondinput thereto, said first input to the second operational amplifierbeing connected to the second input to the first operational amplifier,

a means for generating a third electrical signal,

whose value changes at a prechosen rate corresponding to a desired rateof change of said measured value,

said means having an input and output, said output being connected tothe second input to the first operational amplifier to generate a fourthelectrical signal at the output thereof,

said fourth electrical signal being applied to the input to the meansfor generating the third electrical signal, whereby the latter tendstowards equality with the first electrical signal, and,

means responsive to the output of the second operational amplifier togenerate a process control signal.

5. Apparatus according to claim 4 including switch means operable toconnect the output of said second operational amplifier to the input tothe means for generating the third electrical signal, whereby the latteris caused to equal the second electrical signal.

6. Apparatus according to claim 4 wherein said means for generating thethird electrical signal comprises said first operational amplifier and athird operational amplifier having an output and a first and a secondinput thereto, and a capacitor connected between said output and saidfirst input, the second input being connected to a reference potential,the output of said third operational amplifier being connected to thesecond input to said first operational amplifier, and the output of thelatter amplifier being connected to the first input to the thirdoperational amplifier through a resistor, whereby the electrical chargeon said capacitor changes at a prechosen rate determined by the timeconstant of said resistor and said capacitor, the direction of saidchange being responsive to the sign of the output of the firstoperational amplifier.

7. Apparatus according to claim 5 wherein said means for generating thethird electrical signal comprises said first operational amplifier and athird operational amplifier having an output and a first and a secondinput thereto,v and a capacitor connected between said output and saidfirst input, the second input being connected to a reference potential,the output of said third operational amplifier being connected to thesecond input to said first operational amplifier, and the output of thelatter amplifier being connected to the first input to the thirdoperational amplifier through a resistor, whereby the electrical chargeon said capacitor changes at a prechosen rate determined by the timeconstant of said resistor and said capacitor, the direction of saidchange being responsive to the sign of the output of the firstoperational amplifier.

8. A process control apparatus comprising an electrical circuit having:

a means for generating a first electrical signal representing a desiredvalue of a process variable,

a means for generating a second electrical signal representing ameasured value of said process variable,

a first operational amplifier, with an output and a first and a secondinput thereto, said first input being connected to said first signal,

a second operational amplifier with an output and a first and secondinput thereto, said first input to the second operational amplifierbeing connected to the second input to the first operational amplifier,

a third operational amplifier having an output and a first and secondinput thereto, and a capacitor connected between said output and saidfirst input, the second input being connected to a reference potential,the output of said third operational amplifier being connected to thesecond input to said first operational amplifier, and the output of thelatter amplifier being connected to the first input to the thirdoperational amplifier through a resistor whereby a third electricalsignal whose value changes at a prechosen rate corresponding to adesired rate of change of said measured value is generated at the outputof said third operational amplifier, said third electrical signaltending towards equality with the first electrical signal,

means responsive to the output of said second operational amplifier togenerate a process control signal, and

switch means operable to connect the output of said second operationalamplifier to the first input to the third operational amplifier, wherebythe third electrical signal is caused to equal the second electricalsignal.

9. A process control apparatus according to claim 8, including aplurality of resistors and associated switch means whereby the output ofthe first operational amplifier is selectively connectable through theresistors by the associated switch means to the first input of the thirdoperational amplifier, so that the rate of change of the thirdelectrical signal corresponding to a desired rate of change of saidmeasured value may be preselected.

10. A process control apparatus according to claim 9, wherein the meansresponsive to the output of the second operational amplifier is apolarized relay.

1 l. A process control apparatus according to claim 8 including a fourthoperational amplifier with an output and a first and second inputthereto, and a capacitor and a resistor connected in series between saidoutput and said first input, said first input being connected to theoutput of the third operational amplifier, and said second input to thefourth operational amplifier being connected to the second electricalsignal, whereby the fourth operational amplifier generates a signalwhose magnitude is proportional to the difference between the second andthird electrical signals,

12. A process control apparatus according to claim 9 including a fourthoperational amplifier with an output and a first and second inputthereto, and a capacitor and resistor connected in series between saidoutput and said first input, said first input being connected to theoutput of the third operational amplifier, and said second input to thefourth operational amplifier being connected to the second electricalsignal, whereby the fourth operational amplifier generates a signalwhose magnitude is proportional to the difference between the second andthird electrical signals.

13. A process control apparatus according to claim 9 for controllingtemperature in a process wherein the means for generating the firstelectrical signal includes a thermocouple device.

14. A process control apparatus according to claim 9 wherein the meansfor generating the first electrical signal includes a resistancethermometer.

15. A temperature control apparatus comprising an electrical circuithaving:

a means for generating a first electrical signal representing a desiredvalue of the variable temperature,

a means for generating a second electrical signal representing ameasured value of said temperature,

a first operational amplifier, with an output and a first and a secondinput thereto, said first input being connected to said first signal,

a second operational amplifier with an output and a first and a secondinput thereto, said first input to the second operational amplifierbeing connected to the second input to the first operational amplifier,

a means for generating a third electrical signal,

whose value changes at a prechosen rate corresponding to a desired rateof change of said measured value,

said means having an input and output, said output being connected tothe second input to the first operational amplifier to generate a fourthelectrical signal at the output thereof, said fourth electrical signalbeing applied to the input to the means for generating the thirdelectrical signal, whereby the latter tends towards equality with thefirst electrical signal, and,

means responsive to the output of the second operational amplifier togenerate a temperature control signal.

1. A process control method, comprising the steps of: i. generating afIrst electrical signal representing a measured value of a processvariable, ii. generating a second electrical signal representing adesired value of said process variable, iii. generating a thirdelectrical signal, which changes at a prechosen rate corresponding to adesired rate of change of said measured value of said process variable,iv. comparing said second electrical signal with said third electricalsignal to generate a fourth electrical signal, v. applying said fourthelectrical signal to cause said third electrical signal to change at theprechosen rate towards equality with said second electrical signal, andvi. comparing said first electrical signal with said third electricalsignal to produce a process control signal therefrom.
 2. A processcontrol method according to claim 1, including a further step of causingthe third electrical signal to have an initial value which is equal tothe first electrical signal.
 3. A method of controlling the temperatureof a dyebath, comprising the steps of: i. generating a first electricalsignal representing a measured value of the dyebath temperature, ii.generating a second electrical signal representing a desired value ofthe dyebath temperature, iii. generating a third electrical signal,which changes at a prechosen rate corresponding to a desired rate ofchange of the measured value of the dyebath temperature, iv. comparingsaid second electrical signal with said third electrical signal togenerate a fourth electrical signal, v. applying said fourth electricalsignal to cause said third electrical signal to change at the prechosenrate towards equality with said second electrical signal, and vi.comparing said first electrical signal with said third electrical signalto produce a temperature control signal therefrom.
 4. A process controlapparatus comprising an electrical circuit having: a means forgenerating a first electrical signal representing a desired value of aprocess variable, a means for generating a second electrical signalrepresenting a measured value of said process variable, a firstoperational amplifier, with an output and a first and second inputthereto, said first input being connected to said first signal. a secondoperational amplifier with an output and a first and a second inputthereto, said first input to the second operational amplifier beingconnected to the second input to the first operational amplifier, ameans for generating a third electrical signal, whose value changes at aprechosen rate corresponding to a desired rate of change of saidmeasured value, said means having an input and output, said output beingconnected to the second input to the first operational amplifier togenerate a fourth electrical signal at the output thereof, said fourthelectrical signal being applied to the input to the means for generatingthe third electrical signal, whereby the latter tends towards equalitywith the first electrical signal, and, means responsive to the output ofthe second operational amplifier to generate a process control signal.5. Apparatus according to claim 4 including switch means operable toconnect the output of said second operational amplifier to the input tothe means for generating the third electrical signal, whereby the latteris caused to equal the second electrical signal.
 6. Apparatus accordingto claim 4 wherein said means for generating the third electrical signalcomprises said first operational amplifier and a third operationalamplifier having an output and a first and a second input thereto, and acapacitor connected between said output and said first input, the secondinput being connected to a reference potential, the output of said thirdoperational amplifier being connected to the second input to said firstoperational amplifier, and the output of the latter amplifier beingconnected to the first input to the third operational amplifier througha resistor, whereby the eleCtrical charge on said capacitor changes at aprechosen rate determined by the time constant of said resistor and saidcapacitor, the direction of said change being responsive to the sign ofthe output of the first operational amplifier.
 7. Apparatus according toclaim 5 wherein said means for generating the third electrical signalcomprises said first operational amplifier and a third operationalamplifier having an output and a first and a second input thereto, and acapacitor connected between said output and said first input, the secondinput being connected to a reference potential, the output of said thirdoperational amplifier being connected to the second input to said firstoperational amplifier, and the output of the latter amplifier beingconnected to the first input to the third operational amplifier througha resistor, whereby the electrical charge on said capacitor changes at aprechosen rate determined by the time constant of said resistor and saidcapacitor, the direction of said change being responsive to the sign ofthe output of the first operational amplifier.
 8. A process controlapparatus comprising an electrical circuit having: a means forgenerating a first electrical signal representing a desired value of aprocess variable, a means for generating a second electrical signalrepresenting a measured value of said process variable, a firstoperational amplifier, with an output and a first and a second inputthereto, said first input being connected to said first signal, a secondoperational amplifier with an output and a first and second inputthereto, said first input to the second operational amplifier beingconnected to the second input to the first operational amplifier, athird operational amplifier having an output and a first and secondinput thereto, and a capacitor connected between said output and saidfirst input, the second input being connected to a reference potential,the output of said third operational amplifier being connected to thesecond input to said first operational amplifier, and the output of thelatter amplifier being connected to the first input to the thirdoperational amplifier through a resistor whereby a third electricalsignal whose value changes at a prechosen rate corresponding to adesired rate of change of said measured value is generated at the outputof said third operational amplifier, said third electrical signaltending towards equality with the first electrical signal, meansresponsive to the output of said second operational amplifier togenerate a process control signal, and switch means operable to connectthe output of said second operational amplifier to the first input tothe third operational amplifier, whereby the third electrical signal iscaused to equal the second electrical signal.
 9. A process controlapparatus according to claim 8, including a plurality of resistors andassociated switch means whereby the output of the first operationalamplifier is selectively connectable through the resistors by theassociated switch means to the first input of the third operationalamplifier, so that the rate of change of the third electrical signalcorresponding to a desired rate of change of said measured value may bepre-selected.
 10. A process control apparatus according to claim 9,wherein the means responsive to the output of the second operationalamplifier is a polarized relay.
 11. A process control apparatusaccording to claim 8 including a fourth operational amplifier with anoutput and a first and second input thereto, and a capacitor and aresistor connected in series between said output and said first input,said first input being connected to the output of the third operationalamplifier, and said second input to the fourth operational amplifierbeing connected to the second electrical signal, whereby the fourthoperational amplifier generates a signal whose magnitude is proportionalto the difference between the second and third electrical signals,
 12. Aprocess control apparatus according to claim 9 including a fourthoperational amplifier with an output and a first and second inputthereto, and a capacitor and resistor connected in series between saidoutput and said first input, said first input being connected to theoutput of the third operational amplifier, and said second input to thefourth operational amplifier being connected to the second electricalsignal, whereby the fourth operational amplifier generates a signalwhose magnitude is proportional to the difference between the second andthird electrical signals.
 13. A process control apparatus according toclaim 9 for controlling temperature in a process wherein the means forgenerating the first electrical signal includes a thermocouple device.14. A process control apparatus according to claim 9 wherein the meansfor generating the first electrical signal includes a resistancethermometer.
 15. A temperature control apparatus comprising anelectrical circuit having: a means for generating a first electricalsignal representing a desired value of the variable temperature, a meansfor generating a second electrical signal representing a measured valueof said temperature, a first operational amplifier, with an output and afirst and a second input thereto, said first input being connected tosaid first signal, a second operational amplifier with an output and afirst and a second input thereto, said first input to the secondoperational amplifier being connected to the second input to the firstoperational amplifier, a means for generating a third electrical signal,whose value changes at a prechosen rate corresponding to a desired rateof change of said measured value, said means having an input and output,said output being connected to the second input to the first operationalamplifier to generate a fourth electrical signal at the output thereof,said fourth electrical signal being applied to the input to the meansfor generating the third electrical signal, whereby the latter tendstowards equality with the first electrical signal, and, means responsiveto the output of the second operational amplifier to generate atemperature control signal.